Impact Toughness Testing of Metals...

Impact Toughness Testing of Metals Metals undergo dynamic fracture under rapidly applied loads which are generally produced by impact or by explosive detonation. In comparison to quasi-static loading, dynamic conditions involve loading rates which are higher than those encountered in conventional tensile testing or fracture mechanics testing. Dynamic fracture includes two cases namely (i) a stationary crack subjected to a rapidly applied load, and (ii) a rapidly propagating crack under a quasi-stationary load. In both the cases the material at the crack tip is strained rapidly and, if rate sensitive, can offer less resistance to fracture than at quasi-static strain rates. As an example, values for dynamic fracture toughness are lower than those for static toughness as experienced in the testing of low carbon steels at different temperatures. Many structural components are subjected to high loading rates in service. They also are to survive high loading rates during accident conditions. This makes high strain rate fracture testing is of interest and components are to be designed against crack initiation under high loading rates or designed to arrest a rapidly running crack. Also, since dynamic fracture toughness is normally lower than static toughness, more conservative analysis requires consideration of dynamic toughness. Measurement and analysis of fracture behaviour under high loading rates is more complex than under quasi-static conditions. There are also several different test methods which are used in the evaluation of dynamic fracture resistance. Test methods based on fracture mechanics produce quantitative values of fracture toughness parameters which are useful in design. However, several qualitative methods are also been used in the evaluation of impact energy to break a notched bar, percent of cleavage area on fracture surfaces, or the temperature for nil ductility or crack arrest. These qualitative tests include methods such as...

Mechanical Properties of Steels...

Mechanical Properties of Steels The most important properties of steels which account for their widespread use are their mechanical properties. These properties include a combination of very high strength with the ability to bend rather than break. Different tests have been developed to describe the strength and ductility (a measure of bendability) of steels. A number of these tests which are used to describe the mechanical properties of steels are described below. Tensile testing Tensile testing of steel is a kind of a testing done for the evaluation of the strength of steels. A length of the steel material, usually a round cylindrical rod, is pulled apart in a machine that applies a known force, F. The machine has grips which are attached to the ends of the cylindrical steel rod, and the force is applied parallel to the axis of the rod, as shown schematically in Fig 1. As the force increases, the rod gets longer, and the change in length is represented as delta l (? l), where the symbol delta (?) means ‘a change in’ and the l refers to the original length of the rod. If a force of 50 kg is applied to two rods of the same steel material, where one is thin and the other thick then the thin rod elongate more. To compare their mechanical properties independent of rod diameter, the term ‘stress’ is used. Stress is simply the force divided by the cross-sectional area of the rod. When the same stress is applied to the thin and thick rods, they elongate the same amount, because the actual force applied to the thick rod is now larger than that applied to the thin rod by an amount proportional to its larger area. Because stress is force...